Polyacrylonitrile (PAN) is an acrylic polymer commonly used in many forms. It is manufactured by polymerisation of acrylonitrile monomer (CH2.CHCN) using the solution, suspension or emulsion methods. Typically, a small percentage of co-monomer (such as methacrylate —CH2:CHCOOCH3, or vinyl acetate —CH2:CH—O—CO—CH3) is incorporated into the polymer chain to control crystallinity and hence to modify some physical properties of the polymer. The amount of co-monomer in PAN usually ranges from 0 to 15%. Generally PAN is considered to be a homopolymer (ie: (CH2.CHCN)n) when it contains less than 2% co-monomer.
Molecular weight of PAN can range from 10,000 to 500,000 or more and is closely controlled within the polymerisation process as it has a strong effect on both the efficiency of the polymer production process and the physical properties of the end product.
The basic form of PAN is a fine white powder. In manufacturing PAN materials, this powder is usually dissolved in a solvent (eg, dimethyl formamide, dimethyl acetamide or water-based sodium rhodamide) and the resulting polymer solution (or “dope”) is either cast in the form of a film or spun as a fine fibre. Fibres based on a copolymer PAN are used for a wide variety of textile applications—as knitwear or as a woven fabric for clothing or home textiles. The fibrous form of homopolymer PAN is commonly used for manufacturing a wide variety of woven or felted materials for technical end uses or as a reinforcing element in composite materials (such as brake linings or concrete).
Because of its high level of chemical resistance, homopolymer PAN is also used in various industrial processes and products. Filter materials for both dry and wet applications are commonly made out of PAN.
Fabric filters using PAN fibres for particle collection are seeing widespread use for gas cleaning in many industrial processes. They have a high collection efficiency and produce a gas stream with a very low level of particulates. Operating costs may, however, be considerable as the filter bags must be replaced when either the pressure drop across the filter, or the rate of bag failures, or the level of dust emissions become excessive. The removal of particles of unburnt ash from coal-fired power station exhaust gases is one specific example. Depending on the design of the plant, such a filter material might have to operate at temperatures of up to 135° C. in a flue gas environment containing gaseous oxides of nitrogen and sulphur as well as water, carbon dioxide, nitrogen and oxygen. PAN filter material is well suited to this purpose and has been widely employed in the filters of large power stations of Australia, South Africa, and elsewhere.
One disadvantage of polyacrylonitrile is its susceptibility to shrinkage. The shrinkage of polyacrylonitrile and other polymers used in high extreme conditions (such as high temperature and oxidative conditions) is generally a consequence of oxidation of the polymer. Even in the reduced oxygen environment of combustion flue gas, the rate of PAN oxidation is significant at temperatures above 115° C. Such oxidation causes the PAN to shrink and to become brittle and lose strength. It frequently leads to premature physical failure of the filter media under the combined effects of growing tensile forces and reduced tensile strength.
While it is possible to introduce standard organic antioxidants into PAN, these are quickly decomposed or migrate out of the crystalline structures in the polymer matrix at high temperatures.
Filters for use in high temperature environments are also made from other polymer materials that are, to a greater or lesser degree, also prone to shrinkage due to oxidation. These include polyesters, polyamides, polyolefins such as polypropylene, polyaramids such as Nomex™, fluorocarbon fibres such as polytetrafluoroethylene and polyphenylene-based polymers such as polyphenylene sulphide.